Radioactive well logging method and apparatus



Sept- ,1 I s. KRAsNow ETAL 2,384,840

RADIOACTIVE WELDLOGGING METHOD-AND APPARATUS Filed Oct. 20, 1941 In venzon 844 55544 E); KRAS/VOW L o/v F- CU YYJ idol-1242 Patented Sept. 18,1945 "RADIOACTIVE WELL LOGGING .METHOD AND APPARATUS Shelley Krasnow,New York, N. Y., and Leon F. Curtiss, Montgomery County, Md., assignorsto Geophysical Development Corporation, Washington,.D. 0., acorporationofDelaware Original application April 16, 1937, Serial No.137,380. Divided and this application October 20, 1941, Serial No.415,826

4 Claims.

This invention relates to an improved method and apparatus for measuringradioactivity, and has particular reference to a method andapparatus formeasuring radioactivity in inaccessible locations, such asin'boreholesor at-considerable depths in bodies of water.

One object of the invention is to provide a method and apparatus usefulfor locating deposits of minerals having radioactive properties. Anotherobject of the invention'is to provide an apparatus by which onemaymeasure radioactive properties continuously from the top to thebottom of a borehole, and have both an-immediate .indication. and apermanent record, of the radioactivity. at various depths.

In locating deposits of radioactive minerals it is often thecustom todrill a number of boreholes in localities where such depositsmightexist. .It is further the practice to bring samples or cores of thedrilled material to the surface of the earth, and there examine them forradio activity by well-known methods and apparatus. Thismethodhasseveral drawbacks. First, a deposit of ore may exist close tothe borehole, but not be traversed by it, 'by which the deposit will bemissed. Second, .it is possible to make an error in ascertainingtheexact depth from which a core or'sample has been taken. Finally, 'it israrely possible to bring all of the core to the surface, a certainpercentage always being lost in the drilling or handling.

It is .further known that deposits of petroleum are oftenmarkedlyradioactive as comparedwith the surrounding rock material. This isbelieved to be due to the superior absorptive property of petroleumforradium emanation. Natural gas and ground water are a so known to besomewhat more radioactive than their surrounding rock material. Indrilling for either petroleum or natural gas or ground water, it isdesirable to know-the-exact level at whichthe strata having these aretraversed by the drilled hole. This is often difiicult to determine,particularly when drilling has been done by the rotary" method, in whichthe use of mud under pressure tendsto wall off the strata. Often. too,the drilled'ho-le will be lined with a metallic casing, which casing byaccident or intention may seal off strata having the desired fluid.

It is .an object of the invention described to obviate the difiicultiesmentioned. above.

Reference is bad to theaccompanying draw ing in which:

Figure lshows a form of apparatus for measuring radioactivity at variousdepths in a borehole.

Figure 2 showsa cross-sectional view of the element 1! shown in Figure1, taken across the ;plane.2--2.

onto a-reel l4 operated by a crank i5.

Figure 3 shows the circuit diagram of the apparatus shown in Figure 1.

Figure 4'shows the circuit diagram for another measuring radioactivitywithin a borehole.

A convenient formof the apparatus shown in Figure '1 employs a cartridgell suspended in the borehole by a conducting-cable-IZ. The cablelZpasses over'a measuring wheel '13 and thence A pair of slip-rings Maand Mb fastened to the shaft of thereel I4'havebearing upon them thebrushes l6 and ;I'!. Thesebrushes areconnected through the mediumo-fWires I8, Hi, to a recording element l9. Referring now to Figures 5 and-6, the cartridge i l consists essentially of a-radioactive-sensitivemember '23 mounted at the bottom of apressure-tight cartridge 29. A rackll holds the element 23 and serves further to hold batteries 24, vacuumtube 25, and relay 2:1. Springs 22' serve to prevent violent contact ofthe frame 2| with cartridge 20. A cap 3| is fastened by means of athreaded or other connection 30 onto one end of cartridge 20. Afluid-tightseal is bad by the use of gasket 33. The'wires necessarytoconvey the signals from the cartridge H pass through insulating bushing34 and are looped onto ring 32 andthence pass to the surface. In thisway the wire serves also for raising and lowering member H.

The sensitive element *23' consists essentially of a sealed glass vessel35 which has within it a conducting ring 38 connected to wire 39 passingthrough seal -40. Through the axis of ring 38 there passes anotherconducting member v36, in the form of a wire or filament. This member 36passes through .seal 4| and is further anchored against mechanicalmovement by being fastened to the'bottomof the vessel at-3'l. Container35 is filled with any desired gas such as 2.11131, a pressure which maybe as little as a few centimeters of mercury or as much as atmospheric,and .isthen sealed off, after which it may be used for long periods oftime without further attention. In operation the members 35 and 38 arekept at a high potential relative to each-other bymeans of batteries 46and 44 operating through high resistance leak 41, as shown-in .Figure 3vAsuitable value 'for the voltage of battery 46 is volts; of battery 44,360 volts. The positive end of battery 44 is connected to one side ofthe filament 45 ofvacuum tube 25. Themember 36 is connected to ablocking condenser 42 and thence to the grid 43 of the same vacuum tube.The plate .8 of this tube is connected through relay 2'! to the positiveend of battery 46. The relay 2'1, when de-energized, serves toclose'contacts Hand 29, thus allowing a'current to flow through wires12, slip-rings Ma and Mb, brushes [.6 and I1, electro-magnet 50 andbattery 49. The electro-magnet 5.0 serves :toattract armature 5| whichfurther serves to move pen 52 across the tape 53 keptin constant uniformmotion by means of drum 54 operated by driving means 55. a

The operation of the apparatus is as follows: The members 35 and 38 arecharged at a controlled rate too; high potential relative to each otherby means of the batteries 46 and M operating through leak ll. In thepresence ofradioactive material the gas in the container 35 will bepartially ionized and will thus change the potential of the member 35.This will result in a change of potential of the grid 43 which willreduce the current normally flowing between filament 45 and plate 48 ofvacuum tube 25. This will in turn reduce the current in relay 21sufficiently to allow its armature 28 to be retracted, closing thecircuit between member 28 and contact 29. The output circuit will beintermittently closed by this operation. mittently, conveying responsesconditioned by the intensity of radioactivity. Each operation produces acurrent pulse whose frequency characteristics are modified in accordancewith radioactivity. The closing of this circuit will cause a current toflow through slip-ring Mb, brush I1, battery 49, electro-magnet 58,brush l6, and slipring Ma. The energizing cf electro-magnet 58 willcause armature 5| to move pen 52, causing a break in the line traced ontape 53.

Upon the operation of the circuit in this fashion, the potential ofmember 36 will be restored to its original value, increasing thefilament-toplate current in tube 25, energizing relay 2'! and therebycausing the circuit made by members 28 and 28 to open.

Upon the further ionization of the gas in container 35 the operationabove described will be repeated. Thus, the frequency of the pulsesfinally received by pen 52 will be a measure of the radioactivity of thematerial in the vicinity of member 23. The frequency of the pulsesconstitutes a characteristic or property thereof related to theintensity of radioactivity within the borehole. It will be noted thatthe rays given off by radioactive substances have considerablepenetrating power and can therefore easily penetrate the shield even ifthe latter be made of metal. To reduce the absorption of these rays bythe metal, however, that portion of the cartridge 28 which houses themember 23 is provided with thinner walls than the remainder; aconstruction made possible by the smaller diameter of the said portion.It will be-noted further that even if a metallic casing such as 56exists in the borehole the presence of a radioactive layer such as R.may be noted because of the easy penetration of the rays through thethickness of metal ordinarily employed for easing.

Another type of apparatus is shown schematically in Figllle4. In this,means are provided to charge the electroscope periodically on its chargefalling off by a definite amount. The frequency with which theelectroscope is charged is a measure of the ionization current flowingand thus of the intensity of radiation in the vicinity. The currentobtained has a value dependent upon the intensity of radioactivity.Specifically, the apparatus consists of an ionization chamber having anouter conducting wall 15 into which is fastened a stopcock 1'! throughwhich a suitable gas, such as air, may be passed into the chamber. Anelectrode 14 passes through an insulator 15 and thence into theelectroscope 10. A lamp 81, surrounded by a light tight housing as, andhav- Thus the relay will operate intering a focusing lens 88, casts abeam of light on photo-cell 82. This beam will fall on the photocell 82only if the leaf H is in the discharged position. In such event acurrent passes through cell 82, relay 8|, electro-magnet 84', andenergizing battery 83. The photocell 82 is actuated by the electroscopeacting as a voltage indicator whenever th voltage has changed by apredetermined amount. In the described method of operation, thephotocell is intermittently operated. The responses conveyed from thisintermittently operated element are conditioned by the intensity ofradioactivity. The battery 83 maintains a reference voltage, to keep theelectrodes charged to a definite predetermined potential. The operationof the relay 8! closes contacts 19 and 80, thus causing battery 18 torecharge the electrode M, and thus leaf N. The external voltage ofbattery 18 is repeatedly applied whenever the voltage indicated by leafN has departed from the reference voltage of the battery by apredetermined amount. Whenever this event occurs, pen 84a is caused tomove across chronograph tape 85, and thus produce a kink in the linetraced by the pen. The frequency of these kinks is therefore, a measureof the radioactivity in the vicinity of chamber 16. Thus, the characterof the intermittent pulses is determined by the current flow through theionization chamber, or expressed in other words the characteristics ofthe impulses are determined by the value of the current. The frequencycharacteristics or nature of the pulses are modified by and in definiterelation to the current flowing between the electrodes within theionization chamber. It will also be seen that the characteristics of theimpulses are varied in proportion to the voltage, as indicated by theposition of leaf N. In the operation described, the output circuit willbe intermittently operated by the photocell and will produce currentpulses whose frequency characteristics are modified in accordance withthe radioactivity. This characteristic is a property related to theintensity of radioactivity with in the borehole.

In the types of apparatus shown in Figure 2 and Figure 4, thechronograph and entire recording system may be clock operated andmounted in the cartridgeso that no conducting wires need pass to thesurface. As a further alternative, the motion of the tape may be madenot a function of time, but rather of the position of a measuring wheelsuch as l3. In the apparatus shown in Figure 4, the elements shown as83, 84, 84a, and 85, may be mounted at the surface of the ground,similar to the mounting of element IS in Figure 1; the rest of theapparatus being mounted in a cartridge suitable for lowering to thedesired location. Thus, all of the elements shown in Figure 4, with theexception of members 83, 84, 84a, and 85, would be enclosed within acartridge and lowered into the borehole. These last named elements wouldbe at the surface of the ground as with similar members shown in Figure1.

The apparatus shown in Figure 2 particularly, may be made extremelysensitive to the rays emitted by radioactive substances and so thesometimes faint radioactivity of the petroleum, natural gas and groundwater detected. As has been pointed out previously, this may be done inspite of any covering of mud or of metallic casing intervening betweenthe walls of the borehole and the cartridge I I. It is in fact, possibleto run the cartridge I I inside of the standard drill pipe used inrotary drilling and thus make measurements with a minimum of disturbanceto drilling. Because of the limited absorptive power of the metalscustomarily used for drilling, it will still be possible to detectradioactive rays through the thickness of metal in the drill pipe, oreven through the several inch thickness of the drilling tools.

While, from what has been disclosed above, it is evident that strata maybe differentiated from each other by means of the quantitativedifference in the amount of associated radioactive material, it will beappreciated that strata need not necessarily be widely different intheir associated radioactivity to enable one to differentiate them fromone another. In cases where the associated radioactivities are notconspicuously difierent in conducting measurements from one end of theborehole to the other, valuable information may still be obtained byconsidering the manner in which the radioactivity varies, or phraseddifferently, the function by which radioactive intensity changes as thedepth is altered. This will be found particularly valuable in searchingfor oil deposits. It will be recalled that petroleum deposits in thenatural state have water associated with them. In many cases the waterunderlies the petroleum, and will have a radioactivity markedlydilierent from that of the petroleum itself. Thus if an apparatus, asdescribed above, were lowered past a formation, a sudden change would beobserved in passing from rock to petroleum, another sudden change inpassing from petroleum to water, and still another sudden change inpassing from water to rock. The layers might thus be easily identifiabledespite the fact that their radioactivity may be no greater or less thanthat of most of the rock lining the borehole. It is obvious that anyother means than those shown or described may be used to convey thefrequency of the impulses produced by the apparatus in Figure 2 or inFigure 4, to the surface.

This is a division of our co-pending application,

Serial No. 137,380, filed April 16, 1937.

The scope of the invention is indicated by the appended claims.

We claim:

1. Apparatus for radioactive investigation of drill holes comprising aholder having narrow lateral dimensions and adapted to be inserted inthe drill hole, an ionization chamber mounted upon the said holder, thesaid ionization chamber having means to admit gas within the chamber andto permit closure after the gas has been introduced, means to charge theelements of the ionization chamber, thereby obtaining a current theholder in the drill hole, recording means atthe surface of the earthreceiving the said impulses, the said recording means including amarking member to mark a record, a record chart to receive the saidrecord, and means to move the record chart while recording is takingplace whereby a record of the radioactivity as related to positionwithin the borehole is obtained.

2. In an apparatus for measuring radioactivity in a deep narrowborehole, a holder having narrow lateral dimensions and adapted to beinserted in the drill hole, an ionization chamber mounted thereon, avoltage indicator responsive to the drop of voltage in the ionizationchamber, means actuated by the voltage indicator when the voltage haschanged by a predetermined amount, additional means actuated when thevoltage has changed by the said predetermined amount serving to imposean external voltage to counteract the voltage change, and recordingmeans actuated upon the actuation of the voltage imposing means, servingto record a value indicative of the flow of current through theionization chamber, the recording means including a. record chart, meansto move the said chart while recording is taking place, and a markerserving to mark a record upon the said chart and means to measure theposition of the holder in the drill hole, whereby a record of theradioactivity as related to position within the borehole is obtained.

3. In an apparatus for measuring radioactivity within a borehole, aholder having narrow lateral dimensions and adapted to be inserted inthe drill hole, an ionization chamber mounted thereon adapted to belowered within the borehole and to be responsive to radiations producedWithin the borehole, a source of electrical energy adapted to beconnected to the said ionization chamber and to keep the electrodescharged to a definite predetermined potential, an intermittentlyoperable switch proximate to the ionization chamber, operated by anadditional source of energy and serving to give an output series ofcurrent pulses, the nature of the said currentpulses being modified bythe current within the ionization chamber, and transmitting means fortransmitting the modified pulses to a point of observation, recordingmeans including a record marking member capable of marking a record, arecord chart capable of receiving a record thereon, and means to movethe said chart while recording is taking place in correlation with theposition of the holder in the borehole, thereby obtaining a recordcorrelating depth within the borehole and radioactivity proximate to thesaid depth.

4. In an apparatus for measuring radioactivity in a deep narrowborehole, a mechanically operable switch actuated by a source of energy,a radioactive measuring system actuated by an independent source ofenergy, means to cause the actuation of the switch to produce a seriesof pulses, means connected to the radioactive measuring system to impartto the pulses a property related to the intensity of radioactivitywithin the borehole, all of the aforesaid elements being mounted upon along narrow holder capable of fitting within the borehole, means torecord the characteristics of the pulses as related to depth within theborehole, recording means including a record chart, means to move thechart in correlation with the depth of the holder in the borehole, andmarking means serving to provide indicia upon the chart to indicate thecorrelation between the characteristics of the pulses and depth withinthe borehole.

SHELLEY KRASNOW. LEON F. CURTISS.

